Abstract

Realizing the vast technological potential of patternable block copolymers requires
both the precise controlling of the orientation and long-range ordering, which is
still a challenging topic so far. Recently, we have demonstrated that ordered nanoporous
thin film can be fabricated from a simple supramolecular assembly approach. Here we
will extend this approach and provide a general route to fabricate large areas of
highly ordered polymeric nanodot and nanowire arrays. We revealed that under a mixture
solvent annealing atmosphere, a near-defect-free nanoporous thin film over large areas
can be achieved. Under the direction of interpolymer hydrogen bonding and capillary
action of nanopores, this ordered porous nanotemplate can be properly filled with
phenolic resin precursor, followed by curation and pyrolysis at middle temperature
to remove the nanotemplate, a perfect ordered polymer nanodot arrays replication was
obtained. The orientation of the supramolecular assembly thin films can be readily
re-aligned parallel to the substrate upon exposure to chloroform vapor, so this facile
nanotemplate replica method can be further extend to generate large areas of polymeric
nanowire arrays. Thus, we achieved a successful sub-30 nm patterns nanotemplates transfer
methodology for fabricating polymeric nanopattern arrays with highly ordered structure
and tunable morphologies.

Keywords:

Introduction

Precise fabrication of large areas of ordered nanoscale structures is essential for
microelectronic and information technology, the broad scope of top-down processes,
including conventional immersion lithography, extreme ultraviolet lithography, and
soft lithography have been proposed to meet the demands of devices miniaturization,
these endeavors have enabled the lateral dimensions of devices to be readily shrunk
below 100 nm [1-3]. However, as the lateral dimension goes smaller and smaller, these ‘top-down’ approaches
become extremely difficult and expensive, hence, other methodologies of creating nanostructures
are of great interesting if they can offer advantages in reduced production cost,
smaller feature sizes, and more complex nanopatterns. Nanofabrication via block copolymer
self assembly represents one of the most powerful candidates, and is now taking as
the most promising methodology for next-generation lithography [4-6], mainly due to their intrinsic nanoscale dimensions, facile synthesis, and strict
control of architecture. Ever since the self assembly of block copolymers was introduced
as a powerful ‘bottom-up’ route to well-organized nanostructures decade ago, many
efforts have been devoted: as through chemical modification of the block copolymer
structure to achieve special functionalities, exploring electric fields, and interfacial
interactions to control the orientation, and utilizing solvent induced ordering, salt
complexes, and shear fields to achieve ordered arrays [7-10]. Among which solvent annealing is of particularly beneficial mainly due to their
mild process condition and no need for additional complicated apparatus, and now it
has turn out to be a very simple while robust approach to generate almost defect-free
microphase separation structures in BCP thin films [11-13]. Even more, it appears to be the single possible way for thermal lible systems such
supramolecular block copolymers based on noncovalently bonding. Further research revealed
the use of a co-solvent atmosphere, will enables one to enhancing the ordering process
ever further [14,15]. However, a critical drawback of solvent annealing is that BCP thin film often de-wets
its substrate during solvent exposure as have been already pointed out by several
researchers [16-19]. This makes it is very difficult to obtain uniform and ordered BCP thin film over
a macroscopic area without the direction of additional external fields. In many cases,
realizing the vast technological potential of block copolymers requires both the precise
controlling of the orientation and long-range ordering, however, weakness still remains,
so far, only few works have reported to achieve highly ordered thin film nanotemplates,
and the control of well-organized structures over large scale is still a challenging
topic.

In recent decade, Ikkala and ten Brinke have thoroughly demonstrated that well-ordered
nanostructures in the bulk may be fabricated through supramolecular assemblies (SMA)
of low molecular amphiphiles and block copolymers [20,21]. The amphiphiles can be physically bonded to homopolymers and block copolymers using
noncovalent interactions, this complexation can lead to the formation of supramolcecular
block copolymer which can further assembled into hierarchy nanostructures with various
responsive properties. More recently, we have demonstrated that ordered nanoporous
thin film can be fabricated from a similar approach based on the supramolecular assemblies
of block copolymers PS-PVP and small molecule (2,4-Hydroxybenzeneazo benzoic acid,
HABA) [22-26], the SMA thin films demonstrated hexagonal cylindrical morphology with PS form the
matrix. Solvent annealing in dioxane can enhance the ordering of thin films microphase
separation, following extraction of HABA with selective solvent methanol results in
a nanoporous thin films. The channels can be filled with metal, for example, nickel,
by electrochemical deposition to fabricate an array of ordered metal nanodots or nanowires
with some defects appear in the array due to the nonuniform electrodeposition kinetics
of the metal clusters in nanochannels [23].

In this article, we further investigate the PS-PVP/HABA supramolecular assembly system
in order to achieve a highly ordered morphology and to explore a high definition nanotemplate
replication method for fabrication of highly ordered polymeric nanodots and nanowire
arrays. We will demonstrate that under a mixture solvent annealing atmosphere (with
the dedicating choosing of an additional nitromethane as a selective solvent for the
minor component), a near-defect-free nanoporous thin films with long-range ordering
over a large areas can be achieved. Taking aim at high definition nanotemplate transfer
technique which is another daunting obstacle to the application of the nanoporous
template, we will further show that under the direction of the capillary action and
hydrogen bonding, this ordered nanoporous template can be perfectly transferred, and
thus achieved a methodology for the preparation of highly ordered sub-30 nm polymeric
nanodot and nanowire arrays.

Experimental

Materials

Poly(styrene-block-4-vinylpyridine) (PS-PVP), with Mn PS 4000 g/mol, PVP 5600 g/mol,
Mw/Mn 1.06) for both blocks, was purchased from Polymer Source Inc. A soluble low-molecular-weight
phenolic resin precursor solution was prepared from phenol and formaldehyde using
a basic polymerization method [20]. The final product was redissolved in Ethanol before use as dip coating solution.
2-(4-Hydroxybenzeneazo) benzoic acid (HABA) was purchased from Sigma-Aldrich. Solvents
1,4-dioxane, chloroform, methanol, and dichloromethanes were purchased from Acros
Organics and used as supplied.

Fabrication of Ordered Nanodots and Nanowire Arrays

PS-PVP and HABA (1 mol of HABA and 1 mol of 4-vinylpyridine monomer unit) were dissolved
separately in 1,4-dioxane. PS-PVP solution was slowly added dropwise to HABA solution
while heating to 95 °C in an ultrasonic bath. The resulting solution was kept at least
overnight to complete hydrogen-bond formation. Thin films were prepared by dip coating
from the filtered solutions. Additional 1,4-dioxane/nitromethane mixture solvent vapor
annealing of a thin film was applied to improve the order of nanodomains. Alternatively,
the samples were treated in vapors of chloroform to arrange parallel alignment of
the nanodomains. Nanoporous thin film was fabricated by selective extraction of HABA
with methanol. The nanoporous template was dip coating from the ethanol solution of
phenolic resin precursor, the thin film was sequentially cured by exposure to formaldehyde
gas at 100 °C for 4 h. The cured film was finally pyrolysis at middle temperature
(heating to 450 °C in 2 h and keep 2 h), to remove the PS-PVP and resulted ordered
nanodots arrays (Fig. 1).

Characterization of the Ordered Thin Films

The thickness of the polymer films was measured by a SE400 ellipsometer (SENTECH Instruments
GmbH, Germany) with a 632.8 nm laser at a 70° incident angle. Atomic force microscopy
(AFM) imaging was performed using a Dimension 3100 scanning force microscope (Digital
Instruments, Inc.,) in the tapping mode. Analysis of the AFM images (fast Fourier
transform) was performed with WSxM software (Nanotec Electronica).

Result and Discussions

The as-deposited PS-PVP/HABA thin film form cylindrical phase separation normal to
the substrate, with poor lateral ordering. Due to the thermal liability of hydrogen
bonding, solvent vapor annealing is an elegant approach to promote BCP ordering, in
the previous study, dioxane was chosen as the annealing solvent, here we dedicating
chosen a 1,4-dioxane/nitromethane mixture solvent annealing in order to further enhances
the long-range ordering in relatively short time. After being annealed in a dioxane/nitromethane
mixture solvent for about 24 h, the films were taken out and rinsed in methanol for
5 min to destroy the hydrogen bonding and removes selectively HABA from thin film,
and resulted a nanoporous thin film, the AFM revealed the orientation order is only
of short-range, this is also apparent from a smashed ring in the FFT plot (Fig. 2).

However, further prolonging solvent annealing time to about 72 h in mixture atmosphere
lead to a dramatically increase in the long-range ordering of the thin film, AFM image
clearly identified a near-defect-free ordered arrays of highly ordered hexagonal structure
with all pores oriented perpendicular to the substrate (Fig. 3), the Fourier transform plot of the corresponding AFM phase image is shown in the
inset, the six sharp first-order peaks clearly indicate the presence of a highly ordered
hexagonal structures, this higher order peaks attest to the high degree of order within
the thin film. The mean center-to-center distance of the nanopores based on the AFM
image is about 30 nm. During solvent annealing, PS, and P4VP/HABA, blocks are swelled
by dioxane vapor and tend to organize into a ordered structures, however, this process
is restrained by dioxane in a certain extent due to the good solubility of dioxane
for all the blocks, with the addition of nitromethane which is a good solvent only
for the minor PVP/HABA block, the repulsion between the PS and P4VP/HABA domains is
enhanced, a fast and highly ordered defect-free microphase separation structure is
achieved.

These nanoporous thin films can be used as scaffolds for fabricating organic, inorganic,
and metal nanostructures. Compared with the well studied inorganic and metal nanostructures,
organic (polymeric) nanostructures remains less researched, despite their great potential
in catalysis and membrane separation. Here, we demonstrated that highly ordered phenolic
resin nanodot and nanowire arrays can be prepared through perfect replication of the
ordered nanotemplate with the direction of hydrogen bonding and the capillary action
of nanopores [27]. The above prepared nanoporous thin films were immersed in the phenolic resin precursor/ethanol
solution, the nanopores were immediately filled by phenolic precursors due to the
capillary action of cylindrical nanopores the formation of interpolymer hydrogen bonding
complex between the PVP in the inner pore surface of the nanotemplate and the phenolic
resin. As ethanol is just a good solvent for minor component PVP, while is a nonsolvent
for the matrix composed of PS, thus, the nanoporous templates are maintained during
the dip coating process. When the template was drop from the precursor solution, the
ethanol vaporized and the phenolic resin precursor were maintaining inside the nanopores.
After curation with formaldehyde, the filled nanotemplates were then pyrolysed at
middle temperature (heating to 450 °C in 2 h and keep 2 h), this temperature is enough
to degrade PS-PVP nanotemplate, while the phenolic resin can still maintained. AFM
height image revealed, after pyrolysis, highly ordered discrete phenolic resin nanodots
arrays with uniform diameter can be observed (Fig. 4), of special interest is that the original highly ordered structure of nanoporous
thin film was almost maintained throughout the nanotemplate transfer process and thus
resulted a perfect nanotemplate transformation; the average center-to-center distance
between the nanodots was 30 nm, which was identical to that of the original porous
nanotemplate.

In addition to preparation of nanodots arrays, we further extend this facile nanotemplate
replica method to fabricate polymeric nanowire arrays. A special advantage of the
supramolecular assembled PS-PVP/HABA system we used here is that the orientation of
the phase separation can be reversibly switched from the perpendicular to parallel
orientation and vice versa upon exposure to 1,4-dioxane or chloroform vapor, respectively.
Thus, instead of dioxane vapor, the PS-PVP/HABA supramolecular assembly thin film
was annealed in a saturated chloroform vapor for short time of 15 min to achieve re-alignment.
After rinsing with methanol, the AFM revealed that most of cylindrical microdomains
orientation was re-alignment parallel to the substrate, still it was clear that 15
min is not enough to fulfill the re-alignment process and thus the coexistence of
normal and parallel alignments of nanodomains were observed (Fig. 5a). Followed the same template transfer process, this nanotemplate was dip coating
from phenolic resin precursor solution, and followed by curation and middle temperature
pyrolysis, large areas of short polymeric nanowires with nanodots mixture were formed
(Fig. 5b), which was similar to the original nanotemplate.

Further prolonging the chloroform solvent annealing time to 30 min is enough to fulfill
the re-alignment transformation process, after rinsing with methanol, the AFM revealed
that all the cylindrical microdomains were oriented parallel to the substrate(Fig.
6a), and thus after deposition and middle temperature pyrolysis, a longer nanowire
arrays which was identical to that in the original nanotemplate can be achieved (Fig.
6b).

Conclusion

In summary, we have provided a general route to fabricate highly ordered polymeric
nanodot and nanowire arrays using supramolecular assembled block copolymer thin film
as nanotemplates through the interpolymer hydrogen bonding capillary action. The as-deposited
PS-PVP/HABA thin film formed randomly hexagonally packed cylindrical phase separation
structures, with the dedicating choosing of mixture solvent annealing atmosphere (dioxane
as good solvent for both blocks and an additional nitromethane as a selective solvent
only for the minor component PVP/HABA), a near-defect-free nanoporous thin film with
long-range ordering over a broader range of length scales can be achieved, extraction
of HABA microdomains resulted in highly ordered nanoporous thin films. Under the direction
of interpolymer hydrogen bonding and the capillary action of nanopores, the nanotemplate
can be properly filled with phenolic resin precursor, followed by curation and pyrolysis
at middle temperature which will selectively degrade the PS-PVP block copolymer nanotemplate,
a perfect ordered nanodot arrays replication was obtained, thus resulted in an excellent
and efficient transformation of the nanoporous template to functional polymeric nanodot
arrays. The orientation of the supramolecular assembly thin films can be readily re-alignment
from the perpendicular to parallel the substrate upon exposure to chloroform vapor,
and thus this facile nanotemplate replica method can be further extend to generate
large areas of polymeric nanowire arrays. Thus, we have achieved a successful sub-30
nm patterns nanotemplates transfer methodology for fabricating polymeric nanopattern
arrays with tunable morphology and lateral spacings.

References

Whitesides GM, Grzybowski B:

Science. 2002, 295:2418.

COI number [1:CAS:528:DC%2BD38XisFSls78%3D]; Bibcode number [2002Sci...295.2418W]